For the purpose of explaining you how Evolution works on Morality, lets grant for a second this randomness of moral relativity cultural dependent mkay?
What would happen to a culture where killing all the babies was the relativistic moral cultural standard Cic? Yep it would go extinct!
And what would happen to all other cultures that would protect life, ones better than others? yep the one who did it best would have more offspring, thus it would have the most population.
Now, what would happen as evolution goes on? What genes from what cultures would be left and what genes would be gone? Yep the ones on which the instinct to protect life and cooperate were prevalent.